1. Field of the Invention
The present invention relates to an image forming technique utilizing electrophotography for a printer, a copier machine, a facsimile machine, etc.
2. Description of the Related Art
Among conventional image forming apparatuses utilizing electrophotography is an apparatus in which exposure means forms an electrostatic latent image in each one of a plurality of colors on a photosensitive member, developing means makes toner of the respective colors adhering to the electrostatic latent images, the toner images in the respective colors are each primarily transferred onto an intermediate transfer medium to thereby form a color toner image, which is the toner images in the plurality of colors superimposed one atop the other, on the intermediate transfer medium, and thus formed color toner image is secondarily transferred from the intermediate transfer medium onto a transfer paper at a secondary transfer position which is on the intermediate transfer medium, whereby a color image is obtained.
Known as an image forming apparatus which is of this type and aims at size reduction of a main apparatus section is one that a rotary developer, which comprises a plurality of developer units housing toner in mutually different colors and located in a radial arrangement about a rotation shaft, is driven into rotations about the rotation shaft, one of the plurality of developer units is positioned facing a photosensitive member, an electrostatic latent image on the photosensitive member is accordingly developed with toner, and the toner image is then primarily transferred onto an intermediate transfer medium. In this image forming apparatus, the rotary developer is driven into rotations to thereby switch the developer unit which is to be positioned at a developing position, and development using toner in each color and primary transfer is repeated, so that toner images in the plurality of colors are superimposed one atop the other on the intermediate transfer medium and a color toner image is formed.
A sequence of operations used in an image forming apparatus utilizing electrophotography is, in general, to detect whether there is a transfer paper or not within a transfer paper housing at the transfer paper feed timing, feed the transfer paper which is housed in the transfer paper housing to a transfer paper transportation path when there is a transfer paper, and start an image forming operation (an exposing operation for forming an electrostatic latent image for instance) at predetermined timing after the start of paper feeding. In addition, an apparatus structure is used that on the transfer paper transportation path, there is secondary paper feeding means which is formed by paired registration rollers for instance and performs secondary paper feeding to a secondary transfer position at the timing synchronized to an operation of forming a primary toner image on an intermediate transfer medium. In such an image forming apparatus, a transfer paper fed to the transfer paper transportation path is temporarily held in a bent state by the paired registration rollers which are disposed on the transfer paper transportation path. The front edge of the transfer paper is straightened and securely nipped between the paired registration rollers by the bending force, which prevents a skew and allows preferable secondary paper feeding from the paired registration rollers without deviating from the timing.
A reduction in footprint of an image forming apparatus is desired these days, and therefore, a structure in a popular use is that a transfer paper housing is disposed in a lower part of a main apparatus section, an intermediate transfer medium is disposed above the transfer paper housing, and a transfer paper is transported along a transfer paper transportation path which runs approximately in the vertical direction from the transfer paper housing toward a secondary transfer position. However, in an image forming apparatus having such a structure, since the length of the path from the transfer paper housing to paired registration rollers is short, a time required for a transfer paper to arrive at the paired registration rollers from the start of paper feeding is short and a time that the paired registration rollers hold the transfer paper in a bent state becomes too long in the conventional control sequence as described above, and hence, the transfer paper may be deformed and it may accordingly become impossible to obtain a high-quality transfer image.